In the simplest form, an organic electroluminescent (EL) device is comprised of organic electroluminescent media disposed between first and second electrodes. The first and second electrodes serve as an anode for hole injection and a cathode for electron injection. The organic electroluminescent media supports recombination of holes and electrons that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. A basic organic EL element is described in U.S. Pat. No. 4,356,429. In order to construct a pixelated OLED display device that is useful as a display such as, for example, a television, computer monitor, cell phone display, or digital camera display, individual organic EL elements can be arranged as pixels in a matrix pattern. These pixels can all be made to emit the same color, thereby producing a monochromatic display, or they can be made to produce multiple colors such as a three-pixel red, green, blue (RGB) display. For purposes of this disclosure, a pixel is considered the smallest individual unit, which can be independently stimulated to produce light. As such, the red pixel, the green pixel, and the blue pixel are considered as three distinct pixels.
The simplest pixelated OLED displays are driven in a passive matrix configuration. In a passive matrix, the organic EL material is sandwiched between two sets of electrodes, arranged orthogonally as rows and columns. An example of a passive matrix driven OLED display is described in U.S. Pat. No. 5,276,380. This approach to producing a pixelated display, however, has several disadvantages. First, only a single row (or column) is illuminated at any given time. Therefore, in order to achieve the desired average brightness for a given frame of video, the row should be illuminated to an instantaneous brightness equal to the desired average brightness multiplied by the number of rows. This results in higher voltages and reduced long term reliability compared to a situation where the pixels are capable of being lit continuously for the entire frame. Second, the combination of high instantaneous current and electrodes that are long and narrow, and therefore have high resistance, results in significant voltage drops across the device. These variations in voltage across the display adversely affect brightness uniformity. These two effects become worse as the size of the display and number of rows and columns are increased, thereby limiting the usefulness of passive matrix designs to relatively small, low resolution displays.
To resolve these problems and produce higher performance devices, OLED displays driven by active matrix (AM) circuitry have been shown. In an active matrix configuration, each pixel is driven by multiple circuit elements such transistors, capacitors, and signal lines. This circuitry permits the pixels of multiple rows to remain illuminated simultaneously, thereby decreasing the required peak brightness of each pixel. Examples of active matrix drive OLED displays are shown in U.S. Pat. Nos. 5,550,066, 6,281,634, 6,456,013, 6,501,466, 6,535,185, and 6,753,654.
In order for the light emission to exit the organic electroluminescent device, at least one of the electrodes disposed on either side of the organic electroluminescent medium, such as the anode or cathode, is made to be at least partially transparent. OLED devices are formed on a substrate, such as glass. In one configuration, known as a bottom emitting configuration, the substrate and the electrode between the organic electroluminescent media and substrate, referred to as the lower electrode, are made to be transparent or semi-transparent. In this bottom emitting configuration, the viewer views the display from the side of the substrate. The lower electrode may be constructed, for example, of a conductive film of indium tin oxide (ITO). The other electrode disposed on the opposite side of the organic electroluminescent media, which is referred to as the upper electrode, is typically made to be reflective and highly conductive. Aluminum, Silver, and Magnesium Silver alloys are examples of materials that are useful for this upper electrode.
However, active matrix type OLED displays which are made in a bottom emitting configuration have a problem that the active matrix circuitry, which is typically formed on the substrate prior to the organic electroluminescent media, is not highly transparent to light. Therefore, a portion of the pixel that contains the active matrix circuit components does not emit light. The amount of area that emits light for each pixel relative to the total area of the pixel is known as the aperture ratio (AR). Consequently, the aperture ratio of active matrix type OLED displays which are made in a bottom emitting configuration is relatively low. OLED displays having low apertures ratios must increase the electric current density per unit area through the organic electroluminescent media to achieve the same brightness level as an OLED display having a higher aperture ratio. Driving OLED displays at increase current densities is known to reduce the life time of the device by accelerating luminance efficiency decay over operating time. Also, at increased current densities require the OLED to be driven at increased voltage levels which results in higher power consumption.
To solve these problems, active matrix type OLED displays made in a top emitting configuration have been shown. In a top emitting configuration, light emission generated by the organic electroluminescent media exits the device in the opposite direction of the substrate. Therefore, the active matrix circuitry is not in the path of the light emission and aperture ratio can be increased. Examples of top emitting active matrix OLED displays are shown in U.S. Pat. Nos. 6,737,800 and 6,392,340.
In a top emitting configuration, the upper electrode is made to be transparent or semi-transparent. For example, transparent upper electrodes can be constructed of Indium Tin Oxide (ITO) or similar transparent conductive oxide materials. Also metal films such as aluminum or silver can be used to create a semi-transparent upper electrode if the thickness of the metal is thin, such as, for example, less than 30 nm. However, these transparent and semi-transparent upper electrodes have the problem that they are not highly conductive. That is, since these films are made to be thin or of a material with low conductivity such as ITO, the sheet resistance of the upper electrode is high. High sheet resistance of the upper electrode can result in voltage drops across the upper electrode and across the display, which can result in variations in the luminance output of the pixels. Also, high sheet resistance can result in increased power consumption as well as heat generation.
One approach to improving the conductivity of the upper electrode for a top emitting active matrix type OLED display as proposed in U.S. Pat. No. 6,538,374 is to dispose patterned lines of highly conductive materials, such as aluminum, in electrical contact with the upper electrode in a region between the pixels. However, this approach has the disadvantage that patterning of the lines of conductive material after forming the organic electroluminescent material is difficult to achieve. This is because conventional photolithography with solvent based resist materials often cannot typically be applied without damaging the organic electroluminescent materials. Other methods, such as formation of the lines of conductive material by deposition through a shadow mask are difficult to align, particularly if applied to large substrates.
Another approach to improving the conductivity of the upper electrode for a top emitting active matrix type OLED display is to connect the upper electrode to highly conductive current supply lines provided on the substrate prior to deposition of the organic electroluminescent materials. Such current supply lines can be formed by photolithography methods. Examples of OLED displays where the upper electrode is connected to current supply line located on the substrate are shown in U.S. Patent Application Publications 2003/0146693 A1 and 2004/0160170 A1. In this approach, a region between the pixels is provided with a connection area that is free of the organic electroluminescent materials in order for the upper electrode to make electrical connection to the current supply lines on the substrate. This can be achieved by selectively depositing the organic electroluminescent materials by, for example, use of precision aligned shadow masks. However, such precise patterning and alignment during the deposition by, for example precision aligned shadow masks, is difficult to achieve with the needed accuracy on large substrates or for displays having high resolutions.
Alternately, the organic electroluminescent material can be initially deposited over the pixel emission region and the supply line region without precision deposition alignment and then removed in a region between the pixels by a means such as laser ablation prior to formation of the upper electrode as described in U.S. Patent Application Publication 2004/0253756 A1. However, the organic electroluminescent materials are not typically made to be absorbing of wavelengths commonly used for many lasers, particularly visible and infrared wavelengths. In fact, the organic electroluminescent materials are typically highly transparent at these wavelengths in order to facilitate emission of visible light. This limits the choice of lasers to those having specific ranges of wavelengths such as ultraviolet or near ultraviolet wavelengths or lasers having high power density output, thereby resulting in high manufacturing cost.